The present invention is directed to a phase-difference detector for use with an auto-focus detecting apparatus of a camera.
FIG. 13 illustrates a typical constitution of a conventional auto-focus detecting apparatus of a camera. Description will now be given for this constitution in conjunction with FIG. 9. Disposed behind a film equivalent surface 2 provided in the rear of an imaging lens 1 are a condenser lens 3, a separator lens 4 and a phase-difference detector respectively. The phase-difference detector is composed of linear imaging devices 5 and 6 for optically receiving and photoelectrically converting a pair of images of a subject which are formed by the separator lens 4, and a processing circuit 7 for judging a focusing state on the basis of electric signals generated in respective pixels of devices 5 and 6 in accordance with a distribution of luminous intensities.
The positions of images formed on linear imaging devices 5 and 6 approach an optical axis in a front defocus state where the focused image of the subject is positioned in front of film equivalent surface 2. In a rear defocus state, the position of the focused image moves away from the optical axis. A predetermined position between the front and rear defocus states can be attained in a focusing state. Hence, processing circuit 7 functions to discriminate the focusing state by detecting the position closer to the optical axis for image-formation on the basis of the electric signals from linear imaging devices 5 and 6.
The detection of relative positions of the images formed on linear imaging devices 5 and 6 involves the use of a phase-difference detecting method. Based on this method, correlative values of a pair of images formed on linear imaging devices 5 and 6 are obtained by arithmetic operation pursuant to the following formula (1), and the focusing state is discriminated according to the amount of relative change in value (phase-difference) thereof until the correlative value reaches a minimum (or a maximum). ##EQU1## where L is an integer variable of, e.g., 1 to 9, and corresponds to the amount of relative movement between the images.
The symbol B(K) represents a signal outputted in time-series from each pixel of one linear imaging device 5, while R(K-L-1) designates a signal outputted in time series from each pixel of the other linear imaging device 6. Correlative values H(1), H(2), . . . , H(9) are obtained by performing the arithmetic operation of the formula (1) every time the movement quality L is varied from 1 to 9. Assuming that the focusing state is present when, for instance, the correlative value H(4) is the minimum, if the correlative value in such a position deviates from the minimum, the amount of deviation, i.e., a phase-difference when L=4, is detected as an amount of defocus.
In order to compute the correlative value H(L), as illustrated in FIG. 14, A/D converters 8 and 9 are provided for converting analog signals B(t) and R(t) outputted from linear imaging devices 5 and 6 into digital data Bi and Ri, and also memories 10 and 11 are provided for storing digital data Bi and Ri from all the pixels. After all the data have been stored in memories 10 and 11, the data are sequentially read from memories 10 and 11 at predetermined timings. Subsequently, a digital correlation arithmetic unit 12 incorporating a microprocessor computes the correlative values pursuant to the formula (1).
The following problems are inherent in the above-mentioned conventional phase-difference detector. An A/D converter and a large capacity memory are required for performing the digital correlation arithmetic process. An expensive and high-speed A/D converter is thus needed for effecting the arithmetic operation at a high speed. The correlation arithmetic process involves a multiplicity of repetitions of multiplication and addition, and hence this leads to an increase in rounding error due to restriction in the number of quantization bits of the microprocessor or the like. The increase in rounding error in turn results in a decrease in arithmetic accuracy.